Preparation of magnesium borohydride



Unite PREPARATION OF MAGNESIUM BOROHYDRIDE 2 Claims. (Cl. 23-44) Thisinvention relates to a method for the preparation of magnesiumborohydride. More particularly, it relates to the formation of magnesiumborohydride by the reaction of magnesium hydride with diborane.

The alkaline earth metal borohydrides, such as magnesium borohydride,are well known and are very useful compounds. They are active reducingagents, and are an advantageous source of diborane. They have beenproduced in a variety of ways, but all of these are subject to one ormore defects. It has been proposed to prepare magnesium borohydride bythe reaction of diethylmagnesium with diborane in the presence ofbenzene. This reaction, however, requires excess diborane to avoid theformation of magnesium hydride. Magnesium borohydride has been preparedby Wiberg and Bauer (2. Naturforschg, 5b 397 (1950)) by the reaction indiethyl ether solution of a magnesium dialkyl and diborane at roomtemperature according to the following equation:

The presence of the diethyl ether solvent appears to be necessary ifgood yields are to be obtained in the reaction. The use of diethyl etheras the solvent for this reaction, however, prohibits a large scaleeconomical and safe method of making magnesium borohydride.

In accordance with the present invention, it has been discovered thatmagnesium borohydride can be prepared in good yield by reactingmagnesium hydride and diborane while the magnesium hydride is inadmixture with tetrahydrofuran or a mixture of tetrahydrofuran and up toby volume of a lower dialkyl ether of ethylene glycol, a lower dialkylether of a polyethylene glycol, or a mixture of these dialkyl ethermaterials. The reaction between the magnesium hydride and the diboranewhile the magnesium hydride is in admixture with the tetrahydrofuranappears to proceed according to the following equation:

The dialkyl ethers which can be admixed with tetrahydrofuran to providea reaction medium in accordance with the present invention are those ofthe class RO(CH CH O),,R wherein R and R are alkyl radicals containingfrom 1 to 4 carbon atoms and n is an integer from 1 to 4. Among thesuitable dialkyl ethers which can be used are ethylene glycol diethylether, ethylene glycol dimethyl ether, ethylene glycol di-n-butyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol dimethyl ether, triethyl States Patent 6 i 5 beutilized, if desired.

ice

ene glycol di-n-butyl ether, tetraethylene glycol dimethyl ether,tetraethylene glycol diethyl ether and the like.

The reaction is generally conducted at a temperature within the range of25 to 105 C., although somewhat higher and somewhat lower temperaturescan also In carrying out this reaction, the reaction medium willgenerally contain from about one to about 10 percent by weight ofmagnesium hydride, based upon the weight of the tetrahydrofuran.

The following examples illustrate this invention. the examples the termmoles signifies gram moles.

Example I An experiment was conducted in a 250 ml. round bottomthree-neck flask, fitted with a thermowell. The flask was attached to agas inlet tube, which extended below the surface of the reaction medium,and a water cooled condenser which was connected to a high vacuum systemthrough a trap cooled to 78 C. by immersion in a Dry Ice acetonemixture. A second outlet tube was directly connected to the high vacuumsystem. Agitation of the reactants was accomplishe by a magneticstirrer.

The reaction flask was evacuated and flushed with nitrogen before theaddition of the reactants. The flask was charged with approximately ml.of tetrahydrofuran and 0.0657 mole of magnesium hydridewas added. Themixture was stirred and gradually warmed to approximately 60 C., duringwhich time the apparatus was flushed with nitrogen. After the nitrogenflush was terminated, 0.0370 mole of diborane at the rate of 0.31 ml.per min. was added. During this addition the temperature was maintainedat 61 to 66 C. After the diborane had been completely added, thenitrogen flush was resumed and the mixture was allowed to cool to roomtemperature. This mixture then was filtered in a nitrogen atmospherethrough a sintered glass disc.

The residue remaining on the sintered disc was dried at approximately 80C. while under a vacuum of ap proximately one to five ml. of mercury.The dried residue contained 0.014 mole of magnesium and 0.0388 mole ofboron. This corresponds to 0.014 mole of magnesium borohydride for a72.2 percent yield.

Example 11 This example was conducted similarly to Example I, using asthe reaction medium cc. of a 2.5 to 1 by volume mixture oftetrahydrofuran and diethylene glycol dimethyl ether as the reactionmedium. The magnesium hydride, 0.0305 mole, charged to the reactor, wasadmixed with the reaction medium. Diborane, 0.0467 mole, at the rate of0.44 ml. per minute was passed into the stirred mixture while thetemperature was maintained at 65 to 73 C. In this manner, an 89.5percent yield of magnesium borohydride was obtained.

I claim:

1. A method for the preparation of magnesium borohydride which comprisesreacting magnesium hydride and diborane at a temperature within therange of 25 to C. while the reactants are admixed with a materialselected from the class consisting of mixtures of tetrahydrofuran withat least one compound of the generic formula RO(CH CH,O),,R, wherein Rand R are Patented Mar. 29, 1960 r a alkylradicals containing' from 1 to4 carbon atoms and References'Cited in the fileoi this patent n is aninteger from 1 to 4 in which such compound UNITED TES PA ENTS 2523 285%3 9 z lg g f ggg 2,545,633 Schlesinger Mar. 20, 1951 g f magnesmm M Y ne r m e e 5 2,726,926 Paul Dec. 13, 1955 The method of claim 1 whereinthe reactants are OTHER REFERENCES admixed in a mixture oftetrahydrofuran and diethylene Wiberg et a1.: Chem. Abstracts," vol. 49,column glycol dimethyl ether, said mixture consisting of 2.5 vol 14548,Nov. 10, 1955; article abstracted is by Wiberg umes of tetrahydrofuranand one volume of diethylene 10 et a]. from Z. Naturiorsch.; vol. 106,pages 292-296 glycol dimethyl ether. (1955).

1. A METHOD FOR THE PREPARATION OF MAGNESIUM BOROHYDRIDE WHICH COMPRISESREACTING MAGNESIUM HYDRIDE AND DIBORANE AT A TEMPERATURE WITHIN THERANGE OF 25* TO 105*C. WHILE THE REACTANTS ARE ADMIXED WITH AN MATERIALSELECTED FROM THE CLASS CONSISTING OF MIXTURES OF TETRAHYDROFURAN WITHAT LEAST ONE COMPOUND OF THE GENERIC FORMULA RO(CH2CH2O) R'', WHEREIN RAND R'' ARE ALKLY RADICALS CONTAINING FROM 1 TO 4 CARBON ATOMS AND N ISAN INTEGER FROM 1 TO 4 IN WHICH SUCH COMPOUND REPRESENTS 28.6% TO 50% BYVOLUME OF THE MIXTURE AND RECOVERING MAGNESIUM BOROHYDRIDE FROM THEREACTION MIXTURE.